This work is designed to elucidate the control of fetal cardiac output through studies of the interaction of the peripheral vascular system and the heart, including studies on the activity of several cardiovascular reflexes. We propose to study the effects of hypoxia and chemoreceptor activity on the determinants of venous return. Normal cardiac function will be characterized by cardiac function curves, which will be combined with venous return curves to give an integrated view of the control of fetal cardiac output. After determination of the effects of hypoxia and the effects of the chemo- and baroreceptor systems on both cardiac function and venous return, it will be possible to predict more accurately the response of fetal cardiac output during stress. Also, studies will be made of neonatal cardiac output and mean systemic pressure as they change from their high, in utero values to approach adult values. Overall, these studies will provide information on how the fetus maintains its high cardiac output, how it responds to stresses, and how cardiac output and the determinants of venous return change after birth.